Photographic methods in which silver halide is used provide excellent sensitivity and gradation characteristics compared with other photographic methods, for example, electrophotographic methods and diazo photographic methods, and have widely been used up to now. Among silver halide photographic methods, a number of methods for forming a direct positive image are known. These methods allow one to obtain a positive image by providing uniform exposure, or by using a nucleus-forming agent in developing an internal latent image type direct positive silver halide emulsion in a "surface developing solution", i.e., a developing solution which leaves an internal latent image-forming site in a silver halide grain, without substantially developing the site as described, for example, in U.S. Pat. No. 3,761,276, and JP-B-60-55821 (the term "JP-B", as used herein, means an examined Japanese patent publication).
Such direct positive silver halide emulsions are better than negative type emulsions in terms of their capability to obtain a positive image in only one processing step.
In general, the internal latent image type direct positive silver halide emulsion can be prepared via the steps of mixing a soluble silver salt and a soluble halide in an aqueous solution of gelatin to form a silver halide grain (a core grain), followed by subjecting the core grain to a chemical sensitization, and then to a silver halide deposition for forming a shell, and then carrying out desalting, and a chemical sensitization, if such step is necessary.
The method for preparing an internal latent image type core/shell emulsion which is useful as a direct positive emulsion is described, for example, in JP-A-57-136641 (the term "JP-A", as used herein, means an unexamined published Japanese patent application). This method is characterized by the fact that the surface of the core/shell silver halide grain of the internal latent image type emulsion which contains silver halide grains comprising a silver halide internal nucleus (a core) which has been subjected to i) doping with a metal ion, ii) chemical sensitization, or iii) both of such the processings, and which contains a shell portion which covers at least a light-sensitive site in the internal nucleus, is subjected to chemical ripening in the presence of poly(N-vinyl pyrrolidone), poly(N-vinyl oxazolidone), the copolymer of vinyl alcohol and N-vinyl pyrrolidone, and the copolymer of N-vinyl pyrrolidone and vinyl acetate.
An internal latent image type emulsion which is useful as a direct positive emulsion is described as well in JP-B-52-34213 (U.S. Pat. No. 3,761,276). This emulsion is characterized by the fact that a doping agent is contained in the inside of a silver halide grain, and by the fact that the surface of grain is subjected to chemical sensitization. This is taught as well by U.S. Pat. No. 3,317,322 to Porter et al.
In such preparation methods, various additives used to be used. For example, the sulfur compounds, such as, the thione compounds described in JP-A-53-2408, and JP-A-53-144319, and the thioether compounds described in U.S. Pat. No. 3,574,628 are added during the formation of the silver halide grains to form the silver halide grains having a narrow grain size distribution (that is, a uniform grain size), and a fixed crystal form. These compounds are known as silver halide solvents, and are useful, since the use thereof in forming the internal latent image type core/shell silver halide grains can provide direct positive emulsions having hard gradation, and high sensitivity. Further, the use of sulfur compounds is effective, since the chemical sensitization of an internal nucleus (core) grain for a core/shell type silver halide grain with thione compounds, described in JP-A-55-29829, can effectively accelerate the action of a gold sensitizer which may be used in combination, and thus provide an internal latent image type core/shell silver halide emulsion of high sensitivity.
However, in some cases, additives necessary in the formation of a core grain, and for the chemical sensitization thereof, become unnecessary at the stage of silver halide deposition for forming a shell. For example, in the case of the above silver halide solvents, the grains are dissolved so as to become rounded because of the effect of the solvent on the grains, and, in some cases, the grains cannot be well formed when used in the formation of tabular grains. Further, in carrying out spectral sensitization by adding a sensitizing dye in the final stage for preparing an emulsion, trace amounts of silver halide solvent remaining on the surface of the emulsion grains prevents the sensitizing dye from being sufficiently adsorbent, so that the prescribed effect is not obtained.
The use of a deactivator (for example, hydrogen peroxide) against such a problem is disclosed in JP-A-61-3137, but the deactivator was not useful in cases where it adversely affected photographic performance. Further, there are disclosed in JP-A-61-219948, JP-A-61-219949, JP-A-62-23035, and JP-A-62-240951, emulsions in which chemical sensitizers, such as, gold compounds, and chemical sensitization aids, such as, azaindene, which remain after the preparation of an emulsion, are removed with an adsorbing substance, such as, an ion exchange resin, and an inorganic ion exchanger, but internal latent image type direct positive silver halide emulsions are not described therein. These patents relate to the disclosure of techniques aimed at the removal of unnecessary substances (substances exerting an adverse effects during storage of the emulsions) which remain, after the storage of emulsions, and they do not solve the above mentioned problems relating to emulsion preparation methods.